Microelectromechanical system with piezoelectric film and manufacturing method thereof

ABSTRACT

A method for forming a MEMS device is provided. The method includes forming a stack of layers on a base piezoelectric layer. The stack of layers includes a base metal film over the base piezoelectric layer; a first piezoelectric film over the base metal film; and a first metal film having an opening therein over the first piezoelectric film. The method also includes forming a trench in the stack of layers, wherein the trench passes through the opening in the first metal film but does not expose the base metal film; after forming the trench, forming a spacer structure under the first metal film but spaced apart from the base metal film; after forming the spacer structure, deepening the trench to expose the base metal film; and forming a contact in the trench.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation application of the U.S. applicationSer. No. 16/879,565, filed May 20, 2020, which claims the benefit ofU.S. Provisional Application No. 62/868,638, filed on Jun. 28, 2019, theentirety of which is incorporated by reference herein.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth. In the course of IC evolution, functional density (i.e., thenumber of interconnected devices per chip area) has generally increasedwhile geometry size (i.e., the smallest component (or line) that can becreated using a fabrication process) has decreased. This scaling downprocess generally provides benefits by increasing production efficiencyand lowering associated costs. However, such scaling down has also beenaccompanied by increased complexity in design and manufacturing ofdevices incorporating these ICs, and, for these advances to be realized,similar developments in device design are needed.

Concurrent with advances in functional density, developments inmicro-electromechanical systems (MEMS) devices have led to entirely newdevices and structures at sizes far below what was previouslyattainable. MEMS devices are the technology of forming micro-structureswith mechanical and electronic features. The MEMS device may comprise aplurality of elements (e.g., movable elements) for achieving mechanicalfunctionality. In addition, the MEMS device may comprise a variety ofsensors that sense various mechanical signals such as pressure, inertialforces and the like, and convert the mechanical signals into theircorresponding electrical signals.

MEMS applications include motion sensors, pressure sensors, printernozzles and the like. Other MEMS applications include inertial sensorssuch as accelerometers for measuring linear acceleration and gyroscopesfor measuring angular velocity. Moreover, MEMS applications may extendto sound applications such as micro machined ultrasound transducers andthe like.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a cross-sectional view of a MEMS device, in accordance withsome embodiments of the present disclosure.

FIG. 2 is a magnified cross-sectional view of an area M1 of the MEMSdevice shown in FIG. 1, in accordance with some embodiments of thepresent disclosure.

FIGS. 3-10 are cross-sectional views of a MEMS device in various stagesof a manufacturing method, in accordance with some embodiments of thepresent disclosure.

FIG. 11 is a magnified cross-sectional view of an area M2 of the MEMSdevice shown in FIG. 10, in accordance with some embodiments of thepresent disclosure.

FIGS. 12 and 13 are cross-sectional views of a MEMS device in variousstages of a manufacturing method, in accordance with some embodiments ofthe present disclosure.

FIG. 14 is a flow chart of a method of manufacturing a MEMS device, inaccordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first projection over or on a second projection in the descriptionthat follows may include embodiments in which the first and secondprojections are formed in direct contact, and may also includeembodiments in which additional features may be formed between the firstand second projections, such that the first and second projections maynot be in direct contact. In addition, the present disclosure may repeatreference numerals and/or letters in the various examples. Thisrepetition is for the purpose of simplicity and clarity and does not initself dictate a relationship between the various embodiments and/orconfigurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

One application of microelectromechanical systems (MEMS) devices issound application device. In the sound application device, a membrane ispositioned relative to a cavity. Contact is formed in a trench of themembrane for electrical connection between the MEMS device and othercircuit, e.g. mother board of an electronic apparatus, and signals inrelation to the curvature of the membrane is transmitted to or from thecircuit through the contact. An uneven sidewall of the trench may causea broken of the contact line and adversely effects the transmission ofthe signals. Embodiments of present disclosure provide a MEMS devicewith a trench having a spacer structure formed therein to prevent thecontact from being broken due to uneven side wall of the trench.

FIG. 1 is a cross-sectional view of a MEMS device 10, in accordance withsome embodiments of the present disclosure. In some embodiments, theMEMS device 10 includes a substrate layer 11, a supporting layer 12, anumber of contacts, such as contacts 14, 16 and 18, and a flexible layer20.

The substrate layer 11 may include a silicon substrate in crystallinestructure and/or other elementary semiconductors like germanium having athickness ranging from about 250 micrometers to about 500 micrometers.Alternatively or additionally, the substrate layer 11 may include acompound semiconductor such as silicon carbide, gallium arsenide, indiumarsenide, and/or indium phosphide. The supporting layer 12 is disposedon the substrate layer 11. In example embodiments, the supporting layer12 includes an oxide layer, e.g., thermal or chemical oxide having athickness ranging from about 0.5 micrometers to about 1 micrometer.

In some embodiments, an opening 110 is formed in the substrate layer 11,and an opening 120 is formed in the supporting layer 12. In someembodiments, the opening 120 has a wider width than a width of theopening 110 and may be centrally aligned with the opening 110, and aspace 125 may be formed between the flexible layer 20 and the substratelayer 11. The width of the opening 120 depends on the device design aslong as the space 125 permits a bending movement of the flexible layer20 toward the opening 120. In cases where the MEMS device 10 is used forsound application, the openings 110 and 120 are formed to allow thetransmission of sound waves.

The flexible layer 20 is used to detect a physical wave and producecorresponding signals based on the detected physical wave bypiezoelectric effect. In some embodiments, the flexible layer 20includes a base layer 21 and a stack of piezoelectric films and metalfilms with the piezoelectric films and the metal films being arranged inan alternating manner.

The base layer 21 includes a base piezoelectric film 211 and a basemetal film 212. The base piezoelectric film 211 is formed on thesupporting layer 12. The base piezoelectric film 211 may be or comprise,for example, aluminium nitride (AlN) films and the like and has athickness that is in a range from about 100 Å (angstrom) to about 500 Å.The AlN films with crystal orientation may be used in resonator-basedapplications such as bulk acoustic wave (BAW) and film bulk acousticresonators (FBAR) filters, oscillators and resonating sensors. The basepiezoelectric film 211 may, for example, be formed by chemical vapordeposition (CVD), physical vapor deposition (PVD) or the like.

The base metal film 212 is formed on a side of the base piezoelectricfilm 211 that is away from the supporting layer 12. The base metal film212 may be or comprise, for example, molybdenum (Mo) and the like andhas a thickness that is in a range from about 100 Å to about 500 Å. Thebase metal film 212 is formed overlying the base piezoelectric film 211using suitable deposition techniques, such as atomic layer deposition(ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD)or the like, and the base metal film 212 is patterned according tocircuit design of the MEMS device 10 using suitable photolithographytechniques.

The stack of the piezoelectric films and the metal films is formed onthe base layer 21. In the present embodiment, there are threepiezoelectric films, such as piezoelectric films 22, 24 and 26, andthree metal films, such as metal films 23, 25 and 27, are stacked on thebase layer 21. However, the number of piezoelectric films and the metalfilms in FIG. 1 is given for illustrative purpose, and the MEMS device10 can include any number of piezoelectric films and metal films.

In some embodiments, multiple trenches, such as trenches 13, 15 and 17are formed on the stack of the piezoelectric films and the metal filmsso as to expose the metal films on different levels. In someembodiments, each of the trenches 13, 15 and 17 has different depthrelative to a top surface 202 of the flexible layer 20. For example, thetrench 13 passes through the piezoelectric films 24 and 26 and the metalfilms 23 and 25 and extends into the piezoelectric film 22 to expose thebase metal film 212 of the base layer 21. In addition, the trench 15passes through the piezoelectric film 26 and the metal film 25 andextends into the piezoelectric film 24 to expose the metal film 23.Moreover, the trench 17 extends into the piezoelectric film 26 to exposethe underlying metal film 25. In some embodiments, no trench formed inthe base layer 21, and all the trenches are formed above the base layer21. In some embodiments, the depth of one of the trenches which has thelargest depth (e.g., trench 13) is equal to a distance between the topsurface 202 and the base metal film 212.

FIG. 2 is a magnified cross-sectional view of an area M1 of the MEMSdevice shown in FIG. 1, in accordance with some embodiments of thepresent disclosure. In some embodiments, as shown in FIG. 2, the trench13 has a bottom wall 131 and a slopped side wall 132. The slopped sidewall 132 surrounds the bottom wall 131 and connects the bottom wall 131to the top surface 202 of the flexible layer 20. In some embodiments,the slopped side wall 132 has a step structure and includes a firstslopped segment 133, a first connecting segment 134, a second sloppedsegment 135, a second connecting segment 136, and a third sloppedsegment 137.

The first slopped segment 133 is connected to the bottom wall 131 and isconstructed by a side wall of the piezoelectric film 22 and a side wallof the metal film 23. The first connecting segment 134 connects thefirst slopped segment 133 to the second slopped segment 135 and isconstructed by a portion of a top surface 232 of the metal film 23 thatis not covered by the piezoelectric film 24. The second slopped segment135 is constructed by a side wall of the piezoelectric film 24 and aside wall of the metal film 25. The second connecting segment 136connects the second slopped segment 135 to the third slopped segment 137and is constructed by a portion of a top surface 252 of the metal film25 that is not covered by the piezoelectric film 26. The third sloppedsegment 137 is connected to the top surface 202 of the flexible layer 20and is constructed by a side wall of the piezoelectric film 26 and aside wall of the metal film 27.

In some embodiments, the piezoelectric films of the flexible layer 20have different slopped angles. For example, as shown in FIG. 2, aslopped angle α2 between the second slopped segment 135 and a topsurface 232 of the metal film 23 is greater than a slopped angle α1between the first slopped segment 133 and the top surface 213 of thebase layer 21. In addition, a slopped angle α3 between the third sloppedsegment 137 and the top surface 252 of the metal film 25 is greater thanthe slopped angle α2 between the second slopped segment 135 and the topsurface 232 of the metal film 23. However, it will be appreciated thatmany variations and modifications can be made to embodiments of thedisclosure. The slopped angles α1, α2 and α3 may be within a rangebetween about 5 degrees to about 89 degrees. Any slopped angles arepossible as long as the slopped angles are known and controlled.

In some embodiments, a bottom surface of at least one of metal films 23,25 and 27 of the flexible layer 20 is not entirely covered by theunderlying piezoelectric films 22, 24 and 26, and the bottom surface ofone of the metal films 23, 25 and 27 exposed by the underlyingpiezoelectric films 22, 24 and 26 is covered by a spacer structureinstead. For example, as shown in FIG. 2, the metal films 23, 25 and 27have distal portions 235, 255 and 275 located adjacent to the sloppedside wall 132 of the trench 13. A bottom surface 231 of the metal film23 corresponding to the distal portion 235 of the metal film 23 is notentirely covered by the piezoelectric film 22 and is supported by aspacer structure 31. In addition, a bottom surface 251 of the metal film25 corresponding to the distal portion 255 of the metal film 25 is notentirely covered by the piezoelectric film 24 and is supported by aspacer structure 32. Moreover, a bottom surface 271 of the metal film 27corresponding to the distal portion 275 of the metal film 27 is notentirely covered by the piezoelectric film 26 and is supported by aspacer structure 33. In the present disclosure, the distal portion ofthe metal film is defined as a portion of the metal film with a bottomsurface that is free from contact with the piezoelectric film and is indirect contact with the spacer structure.

In some embodiments, at least two of distal portions 235, 255 and 275have different extension lengths in a direction that is parallel to theextension direction of the base layer 21. For example, as shown in FIG.2, the distal portion 235 has an extension length W1 in a direction D1along which the base layer 21 extends, and the distal portion 255 has anextension length W2 in the direction D1. The distal portion 255 of themetal film 25 (stacked higher) has a greater extension length W2 thanthe extension length W1 of the distal portion 235 of the metal film 23(stacked lower). However, it will be appreciated that many variationsand modifications can be made to embodiments of the disclosure. In someembodiments, the metal film stacked higher has shorter distal portion.For example, in the embodiment shown in FIG. 2, an extension length W3of the distal portion 275 is shorter than both the extension lengths W1and W2 of the distal portions 235 and 255 in the direction D1.

In some embodiments, at least two of the spacer structures 31, 32 and 33have different sizes. For example, as shown in FIG. 2, the spacerstructure 32 located adjacent to the distal portion 255 of the metalfilm 25 has a larger size than the spacer structure 31 located adjacentto the distal portion 235 of the metal film 23. However, it will beappreciated that many variations and modifications can be made toembodiments of the disclosure. In some embodiments, the spacer structurestacked higher has smaller size. For example, in the embodiment shown inFIG. 2, the spacer structure 33 is smaller than both the spacerstructures 31 and 32.

In some embodiments, structural features of the trenches 15 and 17 aresimilar to those of the structure features of trench 13 the descriptionis abbreviated for the sake of brevity.

Referring FIG. 1, the contacts 14, 16 and 18 are respectively formed inthe trenches 13, 15 and 17 and electrically connected to base metal film212 and the metal films 23, 25 and 27 for the electrical connection. Insome embodiments, the contacts 14, 16 and 18 are conformally formed inthe trenches 13, 15 and 17. The contacts 14, 16 and 18 are in directcontact with the slopped side walls of respective trenches 13, 15 and 17and are in direct contact with the spacer structures 31, 32 and 33 (FIG.2).

With reference to FIGS. 3-13, a series of cross-sectional viewsillustrate some embodiments of a method for forming a MEMS device. Themethod may, for example, be employed to form the MEMS device 10 inFIG. 1. While the cross-sectional views shown in FIGS. 3-13 aredescribed with reference to the method, it will be appreciated that thestructures shown in FIGS. 3-13 are not limited to the method and maystand alone without the method. In the present exemplary embodiments,similar processes are also conducted in the MEMS device 10 to form thecontacts 16 and 18, and therefore the processes for forming the contacts16 and 18 are omitted for purpose of brevity.

In the following descriptions, for the purpose of illustration, thepiezoelectric film 22 is referred to as first piezoelectric film, thepiezoelectric film 24 is referred to as second piezoelectric film, thepiezoelectric film 26 is referred to as third piezoelectric film. Inaddition, the metal film 23 is referred to as first metal film, themetal film 25 is referred to as second metal film, and the metal film 27is referred to as third metal film.

As illustrated by the cross-sectional view of FIG. 3, the firstpiezoelectric film 22 is formed on the base layer 21. The firstpiezoelectric film 22 may be or comprise, for example, aluminium nitride(AlN) films and the like and has a thickness that is in a range fromabout 0.3 micrometers to about 0.7 micrometers. The first piezoelectricfilm 22 may, for example, be formed by chemical vapor deposition (CVD),physical vapor deposition (PVD) or the like.

In some embodiments, a plasma treatment 40 is performed after theformation of the first piezoelectric film 22. The plasma treatment 40may include Ar plasma treatment to enhance piezo-efficiency by decreaseAlN roughness. In some embodiments, after the plasma treatment 40, AlNcrystal damage occurs thereby causing a change in crystal structure in atop portion of the first piezoelectric film 22 that is immediatelyadjacent to a top surface 222 of the first piezoelectric film 22. Insome embodiments, the top portion 229 of the first piezoelectric film 22becomes amorphous structure after the plasma treatment. The change incrystal structure may demonstrate different etching rate from that ofthe other region of the first piezoelectric film 22.

As illustrated by the cross-sectional view of FIG. 4, the first metalfilm 23 is formed on the top surface 222 of the first piezoelectric film22. The first metal film 23 may be or comprise, for example, molybdenum(Mo) and the like and has a thickness that is in a range from about 100Å to about 500 Å. The first metal film 23 is formed using suitabledeposition techniques, such as atomic layer deposition (ALD), chemicalvapor deposition (CVD), physical vapor deposition (PVD) or the like.

As illustrated by the cross-sectional view of FIG. 5, the first metalfilm 23 is patterned using suitable photolithography techniques to forman opening 230. In some embodiments, the patterning is performed by anetching process, some other suitable patterning process(es), or anycombination of the foregoing. In some embodiments, the etching processcomprises forming a mask 236 on the first metal film 23, performing anetch into the first metal film 23 with the mask 236 in place, andremoving the mask 236 after the etch. The mask 236 may, for example, beor comprise photoresist, silicon nitride, some other suitable maskmaterial(s), or any combination of the foregoing.

As illustrated by the cross-sectional view of FIG. 6, the secondpiezoelectric film 24, the second metal film 25, the third piezoelectricfilm 26 and the third metal film 27 are formed sequentially over thepatterned first metal film 23 by the similar process forming the firstpiezoelectric film 22 and the first metal film 23, and the formation ofthe flexible layer 20 is completed. In some embodiments, the thicknessof the second piezoelectric film 24 is in a range from about 0.3micrometers to about 0.7 micrometers, the thickness of the thirdpiezoelectric film 26 is in a range from about 100 Å to about 500 Å, andthe thickness of the second metal film 25 and the thickness of the thirdmetal film 27 is in a range from about 100 Å to about 500 Å.

In some embodiments, a protection film 28 is formed over the last metalfilm of the flexible layer 20, such as metal film 27. The secondpiezoelectric film 24 and the third piezoelectric film 26 may be orinclude aluminium nitride (AlN) films and the like. The second metalfilm 25 and the third metal film 27 may be or include molybdenum (Mo)and the like. The protection film 28 may be or include an oxide layer,e.g., thermal or chemical oxide having a thickness ranging from about0.5 micrometers to about 1 micrometer. In some embodiments, amorphousstructure is formed on a top portion 249 of the second piezoelectricfilm 24 by plasma treatment before the formation of the metal film 25.

In some embodiments, as shown in FIG. 6, an opening 250 is formed in thesecond metal film 25, and an opening 270 is formed in the third metalfilm 27 using suitable photolithography techniques. The opening 250 andthe opening 270 are arranged centrally aligned the opening 230 of thefirst metal film 23. In some embodiments, the metal layer stacked higherhas an opening with a greater width than the opening formed in the metallayer which is stacked lower. For example, a width W4 of the opening 230is smaller than a width W5 of the opening 250, and a width W5 of theopening 250 is smaller than a width W6 of the opening 260. The variationof the width of the openings depends on the shape of the trench formedin the flexible layer 20 in the following process shown in FIG. 7A.

As illustrated by the cross-sectional view of FIG. 7A, the flexiblelayer 20 is etched back to form a first trench 13 a via dry etchingprocess with the metal films 23, 25 and 27 served as masks. In someembodiments, the via dry etching process may be conducted with the useof Cl₂ gas or SF₆ gas and is well controlled so that a portion of thelowest piezoelectric film that is immediately adjacent to the base layer21 is remained. Specifically, as illustrated in the embodiment shown inFIG. 7A, the etch may extend into the upper surface of the firstpiezoelectric film 22, for example ranging from 50% to approximately 80%into the depth of the first piezoelectric film 22 in some embodiments.The bottom wall 131 a of the first trench 13 a is distant from the topsurface 213 of the base layer 21. The portion of the first piezoelectricfilm 22 under the bottom wall 131 a of the first trench 13 a may protectthe base metal film 212 from being damaged during the formation of thespacer structures in the following process so as to exhibit a stableelectrode resistance of the base metal film 212.

As illustrated by the cross-sectional view of FIG. 7B, voids 225, 245and 265 are formed on the side wall 132 a of the first trench 13 a. Insome embodiments, the bottom wall 131 a and the side wall 132 a of thefirst trench 13 a is further etched by wet etching process. In someembodiments, at least one of the piezoelectric films 22, 24 and 26 isprocessed with the plasma treatment which makes the top portions 229,249, and 269 of the piezoelectric films 22, 24 and 26 has a higheretching rate than the other portion of the piezoelectric films 22, 24and 26. As a result, the wet etching process removes lateral portions ofthe top portions 229, 249, and 269 of the piezoelectric films 22, 24 and26, thereby leaving voids 225, 245 and 265 on the top portions 229 and249. The voids 225, 245 and 265 exposed a portion of the bottom surfacesof the metal films 23, 25 and 27 relative to the distal portions 235,255 and 275. In other words, the distal portions 235, 255 and 275 of themetal films 23, 25 and 27 is not supported by the underlyingpiezoelectric films 22, 24 and 26. However, it will be appreciated thatmany variations and modifications can be made to embodiments of thedisclosure. In some embodiments, the piezoelectric films 26 is notprocessed with the plasma treatment, and the void 265 is not formed, theside wall of the first trench 13 a corresponding to the thirdpiezoelectric film is a flat surface.

In some embodiments, the remaining portion of the first piezoelectricfilm 22 under the bottom wall 131 a of the first trench 13 a becomethinner after the wet etching in comparison with the piezoelectric film22 shown in FIG. 7A, but the wet etching process is controlled such thatthe underlying base metal film 212 is not exposed by the first trench 13a. The wet etching process may be performed by using H₃PO₄ solutionhaving concentration of 80-85 wt % at a temperature in a range fromabout 120° C. to about 140° C. for about 10 seconds to about 80 seconds.

In some embodiments, the formation of the voids causes a portion of themetal films not being supported by the underlying piezoelectric film andleads to a bending of the distal portions of the metal films. Thebending of the metal films may cause a structure damage of the contactformed in the trench, thereby severely affecting stability of the MEMSdevices 10. To address this problem, spacer structures are filled in thevoids to support the metal films.

Specifically, as illustrated by the cross-sectional view of FIG. 8, aspacer material 30 is formed in the first trench 13 a. The spacermaterial 30 covers the bottom wall 131 a and the side wall 132 a of thefirst trench 13 a. The spacer material 30 is formed using suitabledeposition techniques, such as atomic layer deposition (ALD), chemicalvapor deposition (CVD), physical vapor deposition (PVD) or the like. Thespacer material 30 may be or include thermal oxide or TEOS oxide and hasa thickness of about 500 Å to about 2000 Å. In some embodiments, sincethe spacer material 30 has a good gap filling property such that thevoids 225, 245 and 265 are filled by the spacer material 30.

As illustrated by the cross-sectional view of FIG. 9, the spacermaterial 30 on the bottom wall 131 a and the side wall 132 a of thefirst trench 13 a is removed by an etching process. The etching processis controlled such that the spacer material 30 on the bottom wall 131 aand the side wall 132 a of the first trench 13 a is removed, but thespacer material 30 filled in the voids 225, 245 and 265 is remained. Thespacer material 30 remained in the voids 225, 245 and 265 are referredto as the spacer structures 31, 32 and 33 in the following descriptions.In addition, etching process is controlled such that the first trench 13a does not pass through a portion of the first piezoelectric film 22under the bottom wall 131 a, and the base metal film 212 is not exposedby the first trench 13 a.

In some embodiments, the voids 245 formed in the second piezoelectricfilm 24 (piezoelectric film stacked higher) has a larger size than thevoid 225 formed in the first piezoelectric film 22 (piezoelectric filmstacked lower), and the spacer material entirely filled the voids 225and 245. As a result, the spacer structures 31 and 32 have differentsizes and extension lengths. In some embodiments, the size of the spacerstructure 32 is larger than the size of the spacer structure 31. In someembodiments, the extension length W2 (FIG. 2) of the spacer structure 32is greater than the extension length W1 of the spacer structure 31. Inone exemplary embodiment, the extension length W2 of the spacerstructure 32 is of about 100 nm to about 500 nm, for example, 318 nm,and the extension length W1 of the spacer structure 31 is of about 100nm to about 500 nm, for example, 204 nm. In the cases wherein the void265 is not formed, the spacer structure 33 is omitted. In someembodiments, while the void 265 is not formed, the side wall of thefirst trench 13 a corresponding to the third piezoelectric film 26 isrough, and thus minor unevenness may be formed thereon. The minorunevenness may be filled by the spacer material 30 after the removal ofthe spacer material 30.

As illustrated by the cross-sectional view of FIG. 10, a second trench13 b is formed underlying the first trench 13 a by performing anotherwet etching process. The wet etching process for forming the secondtrench 13 b is controlled such that the second trench 13 b passesthrough the first piezoelectric film 22 to expose the base metal film212. The wet etching process may be performed by using H₃PO₄ solutionhaving concentration of 80-85 wt % at a temperature in a range fromabout 120° C. to about 140° C. for about 30 seconds to about 150seconds. The wet etching process for forming the second trench 13 b maybe performed for a longer time than the wet etching process for formingthe voids 225, 245 and 265. The first trench 13 a communicates with thesecond trench 13 b and collectively referred to as trench 13.

In some embodiments, a surface of at least one of the spacer structures31, 32 and 33 that is adjacent to the slopped side wall 132 of thetrench 13 is protruded from the slopped side wall 132. For example, asshown in FIG. 11, a surface 310 of the spacer structure 31 exposed bythe first piezoelectric film 22 is protruded from the first sloppedsegment 133 of the slopped side wall 132. In some embodiments, at leasta portion of the slopped side wall 132 that is located adjacent to thevoids 225 and 245 is covered by the spacer structures. For example, asshown in FIG. 11, a portion of the first slopped segment 133 of theslopped side wall 132 that is located adjacent to the void 225 iscovered by the spacer structures 31. In some embodiments, a width W7 anda height W8 of the spacer structure 31 are in a range from about 500angstroms to about 2000 angstroms.

In some embodiments, as shown in FIG. 7B, since the distal portions 235and 255 exposed by the voids 225 and 245 are not supported by theunderlying piezoelectric film 22 and 24 before the formation of thespacer structures 31 and 32 in the voids 225 and 245, the distalportions 235 and 255 may become deformed. For example, as shown in FIG.11, the distal portion 235 of the metal film 23 is slightly bentdownward, and the bend angle B1 may be smaller than 5 degrees.

As illustrated by the cross-sectional view of FIG. 12, the contact 14 isformed in the trench 13. In some embodiments, since the voids 225, 245and 265 underlying the metal films 23, 25 and 27 are filled with thespacer structures 31, 32 and 33, the contact 14 is conformally formed inthe trench 13. That is, the contact 14 extends along the first sloppedsegment 133, the first connecting segment 134, the second sloppedsegment 135, the second connecting segment 136, the third sloppedsegment 137 and the third connecting segment 138. As a result, damagesto the contact 14 due to the deformation of the metal films 23, 25 and27 can be prevented, and the stability of the MEMS device 10 isimproved. The contact 14 may be or comprise, for example, aluminiumbronze (AlCu) films and has a thickness that is in a range from about0.5 micrometers to about 1 micrometer.

As illustrated by the cross-sectional view of FIG. 13, the protectionfilm 28 is removed. The protection film 28 may be removed by conductingan etching process. After the removal of the protection film 28, an endportion 141 of the contact 14 is spaced from the top surface 202 of theflexible layer 20. In one exemplary embodiment, the end portion 141 isdistant from the top surface 202 of the flexible layer 20 by a distanceof about 0.5 micrometers to about 1 micrometer.

FIG. 14 illustrates a method of manufacturing the MEMS device 10 inflowchart format S90 in accordance with some embodiments.

At operation S91, a piezoelectric film is formed on a base layer. Insome embodiments, this act can be consistent with all or portions ofFIG. 3, for example.

At operation S92, a plasma treatment is performed over the piezoelectricfilm. In some embodiments, this act can be consistent with all orportions of FIG. 4, for example.

At operation S93, a metal film is formed on the piezoelectric film. Insome embodiments, this act can be consistent with all or portions ofFIG. 5, for example.

At operation S94, the metal film is etched to expose a region of thepiezoelectric film. In some embodiments, this act can be consistent withall or portions of FIG. 5, for example.

At operation S95, the exposed region of the piezoelectric film is etchedto expose a side wall of the piezoelectric film and expose a distalportion of the metal film that is adjacent to the side wall of thepiezoelectric film. In some embodiments, this act can be consistent withall or portions of FIGS. 7A and 7B, for example.

At operation S96, a spacer material is formed on the side wall of thepiezoelectric film and formed on a bottom surface of the distal portionof the metal film. In some embodiments, this act can be consistent withall or portions of FIG. 8, for example.

At operation S97, the spacer material on the side wall of thepiezoelectric film is removed. In some embodiments, this act can beconsistent with all or portions of FIG. 9, for example.

At operation S98, a contact is formed on the metal film and the sidewall of the piezoelectric film. In some embodiments, this act can beconsistent with all or portions of FIG. 12, for example.

Embodiments of the present disclosure conduct a plasma treatment overthe piezoelectric film during a manufacturing process of a MEMS device.Since the roughness of the piezoelectric film is improved, apiezo-efficiency of the MEMS device is enhanced. While voids may beformed on the piezoelectric film due to the crystal damage resultingfrom the plasma treatment, these voids are filled with spacerstructures. Therefore, contact can be conformally formed in the trenchof the piezoelectric film, and a line broken issue of the contact isavoided thereby improving wafer acceptable test (WAT).

In accordance with some embodiments, a method for forming a MEMS deviceis provided. The method includes forming a stack of piezoelectric filmsand metal films on a base layer, wherein the piezoelectric films and themetal films are arranged in an alternating manner. The method alsoincludes etching a first trench in the stack of the piezoelectric filmsand the metal films. The method further includes forming at least onevoid at the side wall of the first trench. In addition, the methodincludes forming a spacer structure in the at least one void. The methodfurther includes forming a contact in the first trench after theformation of the spacer structure.

In accordance with some embodiments, a method for forming a MEMS deviceis provided. The method includes depositing a first piezoelectric filmon a base layer and forming amorphous structures in a first top portionof the first piezoelectric film. The method also includes forming afirst metal film on the first top portion of the first piezoelectricfilm and patterning the first metal film to expose the first top portionof the first piezoelectric film. The method further includes etching afirst trench in the first piezoelectric film. In addition, the methodincludes forming a first void at the side wall of the first trench,wherein the first void is located at the first top portion and forming aspacer structure in the first void. The method further includes forminga contact in the first trench.

In accordance with some embodiments, a MEMS device is provided. The MEMSdevice includes a base layer and a stack of piezoelectric films andmetal films formed on the base layer. The piezoelectric films and themetal films being arranged in an alternating manner. Each of the metalfilms includes a distal portion located adjacent to a side wall of thestack of the piezoelectric films, and at least one of the distalportions of the metal films is free from contact with the piezoelectricfilm. The MEMS device also includes a spacer structure formed adjacentto the at least one of the distal portions that is free from contactwith the piezoelectric film. The MEMS device further includes a contactcovering the side wall of the stack of the piezoelectric films and themetal films and covering a portion of the base layer.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method for forming a microelectromechanicalsystems (MEMS) device, comprising: forming a stack of layers on a basepiezoelectric layer, wherein the stack of layers comprises: a base metalfilm over the base piezoelectric layer; a first piezoelectric film overthe base metal film; and a first metal film having an opening thereinover the first piezoelectric film; forming a trench in the stack oflayers, wherein the trench passes through the opening in the first metalfilm but does not expose the base metal film; after forming the trench,forming a spacer structure under the first metal film but spaced apartfrom the base metal film; after forming the spacer structure, deepeningthe trench to expose the base metal film; and forming a contact in thetrench.
 2. The method of claim 1, wherein forming the contact in thetrench is such that the contact is in contact with the spacer structureand the base metal film.
 3. The method of claim 1, wherein forming thespacer structure is such that a bottom surface of the spacer structureis higher than a bottom of the trench.
 4. The method of claim 1, whereinforming the spacer structure is such that the spacer structure is incontact with a bottom surface of the first metal film.
 5. The method ofclaim 1, wherein the first metal film comprises molybdenum (Mo).
 6. Themethod of claim 1, wherein the first piezoelectric film comprisesaluminum nitride (AlN).
 7. The method of claim 1, wherein the spacerstructure comprises oxide.
 8. A method for forming amicroelectromechanical systems (MEMS) device, comprising: forming a basemetal film over a base piezoelectric film; forming a first piezoelectricfilm over the base metal film; performing a plasma process to the firstpiezoelectric film to form an amorphous structure in a top portion ofthe first piezoelectric film while a bottom portion of the firstpiezoelectric film has a crystal structure; forming a first metal filmover the amorphous structure of the first piezoelectric film; forming atrench passing through the first metal film and the top portion of thefirst piezoelectric film; performing an etching process to etch theamorphous structure of the first piezoelectric film at an etching ratefaster than etch the crystal structure of the first piezoelectric filmsuch that a void is formed in the amorphous structure; and forming acontact in the trench.
 9. The method of claim 8, further comprisingforming a spacer structure in the void prior to forming the contact inthe trench.
 10. The method of claim 9, wherein the spacer structure isspaced apart from the base metal film.
 11. The method of claim 9,wherein forming the contact in the trench is such that the contactcovers the spacer structure.
 12. The method of claim 8, wherein theplasma process is an Ar plasma treatment.
 13. The method of claim 8,wherein forming the trench is performed by using a dry etching processwith Cl₂ gas or SF₆ gas.
 14. The method of claim 8, wherein the etchingprocess is a wet etching process by using H₃PO₄ solution.
 15. Amicroelectromechanical systems (MEMS) device, comprising: a substratelayer having an opening therein; and a flexible layer over the substratelayer and covering the opening, wherein the flexible layer comprises: abase layer; a first piezoelectric film over the base layer andcomprising a top portion and a bottom portion; a first metal film overthe first piezoelectric film; a spacer structure directly between thefirst metal film and the bottom portion of the first piezoelectric film;and a contact in contact with the first metal film, the spacerstructure, and the bottom portion of the first piezoelectric film. 16.The MEMS device of claim 15, wherein the spacer structure is in contactwith a bottom surface of the first metal film.
 17. The MEMS device ofclaim 15, wherein a length of the spacer structure is in a range fromabout 500 angstroms to about 2000 angstroms.
 18. The MEMS device ofclaim 15, wherein the top portion of the first piezoelectric film isamorphous.
 19. The MEMS device of claim 15, wherein the bottom portionof the first piezoelectric film is in crystal structure.
 20. The MEMSdevice of claim 15, wherein a distal portion of the first metal film isbent downward.